1. Field of the Invention
The present invention relates generally to wireless communications and, in particular, to a Long Term Evolution-Advanced (LTE-A) system and power control method of the LTE-A system for controlling cell interference by adjusting transmission power per codeword with a transmission power control offset parameter.
2. Description of the Related Art
Long Term Evolution (LTE), i.e., a next generation wireless communication technology, utilizes Orthogonal Frequency Division Multiplexing (OFDM) for downlink transmission and Single Carrier-Frequency Division Multiple Access (SC-FDMA) for uplink transmission.
In OFDMA, however, Peak to Average Power Ratio (PAPR) is high, which increases a back-off value for an input signal of power amplifier to prevent non-linear distortion of the signal. Accordingly, maximum transmission power is limited as such, resulting in reduced transmission power efficiency. The back-off value limits the maximum value of the transmission power to be less than a value of the power amplifier for guaranteeing the linearity of a transmission signal. For example, if the maximum value of a power amplifier is 23 dBm and a back-off value is 3 dBm, the maximum transmission power is limited to 20 dBm.
When OFDMA is used as downlink multiplexing technology, there are no problems because the transmitter belongs to a base station having no power limit. However, if OFDMA is used as uplink multiplexing technology, because the transmitter belongs to a User Equipment (UE) that has a very limited in transmission power, the coverage of the base station reduces due to the limit of maximum transmission power of the UE. Therefore, in order to increase uplink coverage, SC-FDMA has been adopted for uplink multiplexing technology of LTE as the 4th generation wireless communication standard of 3rd Generation Partnership Project (3GPP).
With of the increase use of multimedia services in the wireless communication environment, many researchers are focusing on techniques for achieving high speed transmission. For example, Multiple Input Multiple Output (MIMO) is one of the key techniques for increasing spectral efficiency and link reliability for high speed transmission.
MIMO technology uses multiple antennas to increase channel capacity in a limited frequency resource. Logically, the channel capacity of MIMO increases in proportion to the number of antennas being used. In order to transmit data efficiently in a MIMO system, the data is encoded in advance, which is commonly referred to as “precoding”. The data precoding rule can be expressed by a precoding matrix, and a set of precoding matrices is commonly referred to as a “codebook”.
In LTE-A, a MIMO technique using a precoding matrix is recommended as an uplink transmission technology for improving system performance in multiuser environment as well as single-user environment.
In LTE uplink, event-triggered power control is used for Physical Uplink Shared Channel (PUSCH). In PUSCH, Transmit periodic Power Control (TPC) feedback is not required. PUSCH transmission power at subframe i PPUSCH(i) can be expressed as shown in Equation (1).PPUSCH(i)=min{PCMAX,10 log10(MPUSCH(i))+PO—PUSCH(j)+α(j)·PL+ΔTF(i)+f(i)}[dBm]  (1)
In Equation (1), PCMAX denotes a maximum transmission power according to the UE power class, and MPUSCH(i) denotes a number of Resource Blocks (RBs) as the PUSCH resource assigned in the subframe i. The transmission power of the UE increases in proportion to MPUSCH(i). PL denotes downlink path-loss measured at the UE, and α(j) denotes a scaling factor, which is determined at higher layers in consideration of a difference between uplink and downlink channels established by a cell formation. PO—PUSCH(j) can be expressed as shown in Equation (2).PO—PUSCH(j)=PO—NOMINAL—PUSCH(j)+PO—UE—PUSCH(j)  (2)
In Equation (2), PO—NOMINAL—PUSCH(j) denotes a cell-specific parameter signaled by a higher layer, and PO—UE—PUSCH(i) denotes a UE-specific parameter transmitted through Radio Resource Control (RRC) signaling. A Modulation and Coding Scheme (MCS) or Transport Format (TF) compensation parameter ΔTF(i) can be defined as shown in Equation (3).
                                          Δ            TF                    ⁡                      (            i            )                          =                  {                                                                      10                  ⁢                                                            log                      10                                        ⁡                                          (                                                                        2                                                                                    MPR                              ⁡                                                              (                                i                                )                                                                                      ·                                                          K                              S                                                                                                      -                        1                                            )                                                                                                                                        for                    ⁢                                                                                  ⁢                                          K                      S                                                        =                  1.25                                                                                    0                                                                                  for                    ⁢                                                                                  ⁢                                          K                      S                                                        =                  0                                                                                        (        3        )            
In Equation (3), KS is a cell-specific parameter transmitted through RRC signaling.
MPR(i) is calculated by Equation (4).
                              M          ⁢                                          ⁢          P          ⁢                                          ⁢                      R            ⁡                          (              i              )                                      =                              T            ⁢                                                  ⁢            B            ⁢                                                  ⁢                          S              ⁡                              (                i                )                                                                                                          M                  PUSCH                                ⁡                                  (                  i                  )                                            ·                              N                SC                RB                            ·              2                        ⁢                          N              Symb              UL                                                          (        4        )            
In Equation (4), TBS(i) is a transport block size of subframe i, and MPUSCH(i)·NSCRB·2NSymbUL denotes a number of Resource Elements (REs) within the subframe. MPR(i), which is calculated by Equation (4) indicates a number of information bits per RE.
If KS=0 and MPR(i)=0, then the uplink channels are not compensated for MCS.
If KS=1.25, only 80%
  (            1              K        S              =    0.8    )of the uplink channels are compensated for MCS.
The current PUSCH power control adjustment state is given by f(i), which is defined by equation (5).f(i)=f(i−1)+δPUSCH(i−KPUSCH),  (5)
In Equation (5), δPUSCH is a UE-specific parameter included in a Physical Downlink Control Channel (PDCCH) transmitted from the base station to the UE and can be a TPC value, and KPUSCH in δPUSCH(i−KPUSCH) denotes a time interval between receiving δPUSCH value and using it in the transmission subframe.
The δPUSCH dB accumulated values signaled on PDCCH with DCI format 0 are [−1, 0, 1, 3]. The δPUSCH dB accumulated values signaled on PDCCH with DCI format 3/3A are [−1, 1] or [−1, 0, 1, 3].
Rather than δPUSCH accumulated values as shown in Equation (5), δPUSCH dB absolute values can be used as shown in Equation (6).f(i)=δPUSCH(i−KPUSCH)  (6)
In Equation (6), the δPUSCH dB absolute values signaled on PDCCH with DCI format 0 are [−4, −1, 1, 4].
FIG. 1 is a flowchart illustrating a base station-assisted UE power control process in a conventional LTE system.
Referring to FIG. 1, the base station determines whether to use PDCCH or RRC signaling to transmit power control parameters to the UE in step 101.
If PDCCH is selected (e.g., δPUSCH), the base station sends the power control parameters to the UE on PDCCH in step 102. However, if RRC signaling is selected (KS), the base station sends the power control parameters using RRC signaling in step 103.
In step 104, the base station measures Signal-to-Interference plus Noise Ratio (SINR) using Sounding Reference Signal (SRS) transmitted by the UE.
In step 105, the base station updates the power control parameters in consideration of the received signal strength and the interference of the signal, which is transmitted by the UE, to neighbor cells. The updated parameters are transmitted to the UE on the channel determined in step 101.
In the LTE-A system, two codewords are transmitted on the PUSCH with MIMO. Accordingly, applying the convention power control method designed for the LTE uplink using a single antenna and single codeword to the LTE-A system increases cell interference due to the excessive transmission power of the UE and early transmission power shortage.